This invention relates to a pre-amplifier for use in a three electrode single tube type color television camera capable of adjusting the gain of the amplifier without causing any cross talk.
Color television cameras are classified into a three tube type camera utilizing three image pick-up tubes for red, green and blue colors, and a three electrode single tube type camera in which three signal electrodes for red, green and blue colors contained in a single tube are divided into stripes and a plurality of sets of the stripes are arranged alternately.
According to one example of a circuit for processing the output signal of a three tube type color television camera, a red signal produced by a red image pick-up tube is amplified by a pre-amplifier, adjusted its gain with an attenuator and then applied to a process amplifier. A green signal generated by a green pick-up tube and a blue signal produced by a blue pick-up tube are also applied to a respective process amplifiers through respective pre-amplifiers and attenuators. The electric signals shaped by respective process amplifiers are applied to a common encoder to be converted into NTSC color television signals.
Since, owing to the variation in color temperatures, the ratio of electric signals produced by respective image pick-up tubes is not 1:1:1, the gains are adjusted by associated attenuators such that when a white object is picked up, the ratio of input signals of respective process amplifiers would be 1:1:1.
However, when this circuit is used for a three electrode single tube type color television camera, there is a problem of causing cross talks.
More particularly, in an equivalent circuit of a three electrode single tube type image pick-up tube for use in a color television camera, the red color signal generated by a current source of the image pick-up tube for red color, the green color signal generated by a current source for green color and the blue color signal generated by a current source for blue color flow through associated load resistors equivalently representing the signal processing circuit. However, there are electrostatic capacitances between three electrodes of the single tube type camera. This is equivalent to connecting capacitors between the electrodes. When the terminal voltages across respective load resistors are not equal, currents corresponding to the voltage differences flow to the circuits of different colors through the capacitors, thus causing cross talks. Let us denote the signal current from the source for red color by I.sub.R, the signal current from the source for green color by I.sub.G and the signal current from the source for blue color by I.sub.B. Further, let us denote the resistance values of respective load resistors by R.sub.R, Z.sub.G and Z.sub.B. The condition necessary to prevent cross talk is expressed by the following equation. EQU I.sub.R .multidot.Z.sub.R =I.sub.G .multidot.Z.sub.G =I.sub.B .multidot.Z.sub.B
In other words, in order to prevent cross talk, it is necessary to adjust the input impedance of the pre-amplifier of the output signal processing circuit such that the input impedance is inversely proportional to signal currents from respective current sources.
One example of the solution to this problem is disclosed in Japanese Patent Application Laid-Open No. 65123/1975.
According to this solution, however, for the purpose of effecting white balance while at the same time preventing cross talk, it is necessary to adjust the output level of the pre-amplifier concurrently with the adjustment of its input impedance. Such adjustments are complicated and not practical.